Method and device for detecting misfiring of internal combustion engines

ABSTRACT

A method for detection of misfiring of an internal combustion engine, wherein the torque of the internal combustion engine is measured with the aid of a torque sensor mounted on the crankshaft of the internal combustion engine and wherein a torque signal is sampled speed-synchronously with the aid of a sensor. A digital filtering of the torque signal is carried out with a digital filter with a finite impulse response which blocks or damps a selection of integer or half-integer multiples of the speed of the internal combustion engine. The selection of speed multiples is substantially dependent on the number of cylinders of the internal combustion engine.

TECHNICAL FIELD

The present invention relates to a method and to a device for detectionof misfiring of an internal combustion engine, in which the torque ofthe internal combustion engine is measured by means of a torque sensorarranged on the crankshaft of the internal combustion engine and where atorque signal is sampled speed-synchronously.

BACKGROUND OF THE INVENTION

Misfiring of internal combustion engines is an operational disturbancewhich implies that the fuel/air mixture compressed in the cylinder isnot completely burnt, or not burnt at all, for example because of afailing ignition spark (Otto engine) or disturbances during injection(Diesel engine). It therefore results in reduced power of the internalcombustion engine, increased wear on components included and, inaddition, entails increased emission of environmentally harmfulsubstances as, for example, unburnt hydrocarbon compounds. Misfiringalso contributes to reduction of the service life of the exhaust gascatalytic converter of the internal combustion engine. Detecting andindicating misfiring is, therefore, of great importance for an increasedservice life of internal combustion engines. Detection also permits apossibility of reducing the environmental influence and of complying ina better way with the increasingly stricter demands made by thelegislation on emission of various pollutants from internal combustionengines.

One way of defining misfiring is based on the work which is carried outin each cylinder during a work cycle, which, in the case of aconventional four-stroke Otto engine or diesel engine, consists of twocrankshaft turns (720°). If the work for a cylinder is insignificant inrelation to the amount of fuel present in the cylinder, it can be saidthat, by definition, misfiring has taken place. As a measure of thiswork, a so-called indicated mean effective pressure (IMEP) is oftenused, which is obtained by dividing the measured work by the pistondisplacement of the cylinder.

Misfiring can be detected in a plurality of different ways.

A direct measurement of the cylinder pressure in each individualcylinder may, of course, be used for detecting misfiring. Because of thesevere environment in which the necessary pressure sensors are tooperate, the method is costly and has substantially been used only invery large internal combustion engines. Since the method is based on acomparison of the output signals from several different sensors, varyingaging between the sensors may lead to problems with the reliability ofthe method after a long time. In addition, it is necessary to haveaccess to the crank angle for each cylinder with great accuracy in orderfor the calculation of the work carried out to become reliable.

Another method for detecting misfiring comprises measuring theelectrical conductivity of the gas present in the cylinders of theinternal combustion engine. This measurement may be performed by usingthe spark plug of the respective cylinder as measuring electrode. Such amethod is described, for example, in U.S. Pat. No. 5,207,200. During amisfire, the gas in the cylinder becomes colder than after a successfulignition, and the gas therefore has a lower conductivity during amisfire. An advantage of this method is that an electrical quantity maybe measured directly without needing access to a separate sensor.However, the method places special demands on the design of the ignitionsystem of the internal combustion engine. As an example, it may bementioned that special spark plugs are required as well as a separateignition coil for each cylinder. Problems may arise due to varying agingof the spark plugs and the reliability during no-load operation,acceleration and braking is not good. This, method makes demands for avery high calculation capacity. Additional methods for detectingmisfiring are based on measurement of the engine speed, see, forexample, U.S. Pat. No. 5,216,915 and U.S. Pat. No. 5,278,760. Toequalize the power of the normal torque fluctuations of the internalcombustion engine, the internal combustion engine is normally providedwith a flywheel which equalizes the speed fluctuations. This impliesthat the variations which arise in the speed of the internal combustionengine because of misfiring will also decrease. A signal which may beobtained in different ways, and corresponding to these speed variations,will therefore have a low signal level. To this is to be added the factthat the signal is influenced by disturbances from irregularities in thebase and a flywheel which is not perfectly balanced. Disturbances mayfurthermore arise in different operating situations such as duringacceleration, changing, disengagement of the clutch etc. The combinationof a high speed and a low load is an operational situation whichinvolves problems difficult to handle because periodic inertial forces,generated primarily by the movements of the connecting rods and thepistons, dominate in relation to the force which is developed in andinfluences the pistons of the cylinders during faultless firing. Insummary, the low signal level and the large number of disturbancesources imply that methods based on measurement of the engine speedrequire calculation-intense methods of evaluation and have a limitedreliability.

SUMMARY OF THE INVENTION

The invention relates to a method and a device for detecting misfiringof an internal combustion engine wherein the torque of the internalcombustion engine is measured with the aid of a torque sensor arrangedon the crankshaft of the internal combustion engine and wherein a torquesignal is sampled in a speed-synchronous manner.

The torque signal is filtered with a digital filter with a finiteimpulse response which greatly blocks or damps a selection of integer orhalf-integer multiples of the speed of the internal combustion engine,where the selection of speed multiples depends, among other things, onthe number of cylinders of the internal combustion engine.

By measuring the torque on the crankshaft with a torque sensor, it is,in principle, possible to see the effect of each individual firing andto detect variations between the cylinders and subsequent firings in thesame cylinder. When the firing in a cylinder completely fails, no network is generated by that cylinder during its compression and expansionphase, which must also in some way manifest itself in the measuredtorque.

To measure the torque generated by the engine as accurately as possible,it is suitable to measure this torque as close to the crankshaft aspossible, preferably between the crankshaft and the flywheel. In thisway, the effect of torsional oscillations on the driving rope andvibrations when the car is driving on an uneven road surface arereduced.

The difficulty is to extract from the torque signal a measure percylinder and work cycle which correlates well with the work carried outby the respective cylinder during the cycle mentioned, and hence makespossible detection according to the definition of misfiring. Inaddition, the method should preferably be inexpensive to implement.

However, the instantaneous value of the output signal of the torquesensor is not, by itself, a good instrument for detecting misfires,except possibly at very low speeds, when the influence of resonantoscillations is small. During resonance, the amplitude of the resonantoscillations may exceed the change of the torque on account of failingfiring by a factor of ten.

The method according to the invention for obtaining a measure ofmisfiring comprises forming a weighted mean of the torque signal withinan interval around the firing time for each cylinder. With a suitablychosen weighting function, an indicated mean effective torque (IMET) maythus be formed, which correlates well with the work carried out by thecylinder, independently of resonant oscillations. This is achieved bydetermining the weighting function in a way which makes the weightedmean independent of cyclic variations with definite periodicities of thesignal.

A more detailed explanation of how the above-mentioned weightingfunction is to be formed, and how the method functions, is based on theproperties of digital filters with a finite impulse response filter, orFIR filter. These are described more closely in a number of books on thesubject, among other things in Handbook of Digital Signal Processing,Ed. by Douglas F. Elliot, Academic Press 1987, Chapter 2, page 55 etseq.

When filtering with a digital FIR filter, the input signal is sampled atdiscrete points of time. The output signal from the filter then consistsof a weighted mean of a number of samples from the input signal to thefilter. The weighting function constitutes the so-called impulseresponse of the filter. For each new sample point in the output signal,the mean value formation of the input signal is carried out displaced byone sample point. In this way, new values from the input signal willconstantly give rise to the output signal. Mathematically, this methodimplies that the input signal is convoluted with the impulse response ofthe FIR filter. The impulse response completely determines theproperties of the filter.

A filter with a rectangular impulse response, a so-called comb filter,has, for example, the property that all the frequencies which aremultiples of the inverse of the duration of the impulse response arecompletely eliminated, and adjacent frequencies are damped in acharacteristic way. Filtering with such a filter means that the runningmean value of the input signal during a certain period of time, or of acertain number of samples corresponding to the duration of the impulseresponse, is continuously formed during a certain time.

For the use during misfiring detection, it is sufficient to carry outthe FIR filtering at only one definite crankshaft angle in relation tothe upper dead center of the respective cylinder at the beginning of thepower stroke, where a suitable crankshaft angle is chosen according toexperience to obtain the clearest reading. This means that the actualcalculation only needs to be carried out N times per two turns of thecrankshaft, where N is the number of cylinders of the engine.

The dynamic behavior of the crankshaft in an internal combustion engineis very complex and may be regarded as a number of differentpart-processes which, taken together, give rise to a characteristicoscillation behavior superimposed on the speed of the output shaft. Oneway of describing this oscillation behavior for different loads andspeeds is to use so-called tracking curves, which show the oscillationamplitude divided into its different harmonic components as a functionof the speed.

Depending on the number and location of the cylinders in the engineblock, characteristic oscillation behaviors arise for each internalcombustion engine.

The torsional oscillation behavior of a five-cylinder in-line engineprimarily comprises the combustions exciting oscillations constitutingthe 2.5^(th) tone to the speed and its harmonics. This is due to thefact that the internal combustion engine ignites five times per twoturns. This excitation results in resonant oscillations where thedifferent harmonics arrive to different degrees at different speeds withtheir maximum at the frequency where the frequency of the harmoniccorresponds to the resonant oscillation of the internal combustionengine. At certain speeds, oscillations with frequencies equal to twoand three times the speed are also of importance and give rise todifferent effects.

In a four-cylinder in-line engine, for example, the second tone to thespeed and its harmonics are completely predominant.

In the same way, the torsional oscillation behavior for engines withother numbers of cylinders and other cylinder devices may be derivedtheoretically or be measured experimentally. The theoretical derivationis most suited for in-line engines with equidistant ignitions, theexperimental measurement for engines with a more complicated structure,for example V-engines.

To obtain the indicated mean effective torque independently of itstorsional oscillations, according to the invention a weighting function(impulse response) is chosen which filters away oscillations at thesefrequencies.

The most common form of digital filtering involves sampling the signalto be filtered at definite times separated by a constant time interval,the so-called sample period. This method is suitable if it is desired tofilter signals with a definite frequency content.

To make possible the use of the same filter function for detectingmisfiring independently of the speed of the engine, according to theinvention, use is made of the fact that disturbing torsionaloscillations due to resonance only occur for such frequencies where theresonant frequency corresponds to a frequency which is excited by theengine. As described above, these excited frequencies are coupled to thespeed of the engine.

By sampling instead the torque signal at times separated by an intervalbased on a certain rotation of the crankshaft, so-calledspeed-synchronous sampling, properties of the transfer function of asubsequent digital filter are obtained where the properties are coupleddirectly to the speed of the engine instead of to an absolute frequency.Such a method is therefore suitable for detecting misfiring.

For a four-cylinder engine, oscillations are excited with a frequencyequal to multiples of twice the speed to the engine. A filter with arectangular impulse response with a duration corresponding to half arevolution, which operates on a speed-synchronously sampled signal,eliminates these very frequencies independently of the speed of theengine. In addition, it is very simple and computationally effective toimplement.

For a five-cylinder engine, a filter which eliminates frequencies equalto 2, 2.5 and 3 times the speed of the engine is required.

To reduce the crosstalk between the cylinders, and accuracy problemsassociated therewith, it is important that the filter used has animpulse response with as short an extent as possible. In addition, ashort impulse response reduces the required number of multiplicationsand additions during the filtering.

A filter with a minimum length suitable for detecting misfiring infive-cylinder four-stroke engines is obtained according to the inventionby combining filters with rectangular impulse responses, comb filters,corresponding to a 180°, 144° and 120° rotation of the crankshaft.During continuous filtering for each sample point, the mostcomputationally effective way is to apply these three rectangularfilters one after the other on the signal. For filtering at few selectedsample points per revolution only, the filtering is instead performed byforming a combined impulse response for these filters where the impulseresponses for each comb filter according to known technique areconvoluted with one another. The impulse response thus obtained is shownin FIG. 2. The spectrum for the transfer function for such a filter isshown in FIG. 3.

In a corresponding way, a suitable filter for six-cylinder four-strokeengines is obtained by combining comb filters corresponding to 120° and80°, respectively. This eliminates frequencies equal to 3 and 4.5 timesthe speed of the engine. For eight-cylinder four-stroke engines, asuitable filter is obtained by combining comb filters corresponding to90°, 72° and 60°. This eliminates frequencies equal to 4, 5 and 6 timesthe speed of the engine.

The disadvantage of mean-value formation according to theabove-mentioned technique is the relatively extensive calculationcapacity which, despite the small number of filterings per twocrankshaft turns, is required to carry out the number of multiplicationsand additions which is necessary. The number of required multiplicationsmay, however, be eliminated completely and the number of additions begreatly reduced if, instead of forming a weighted mean according toconventional filter technique, an alternative method according to theinvention is used.

According to this method, certain of the collected values are selectedaccording to a definite pattern, for example 16 or 32 values, placedwithin an angular interval, which are then summed. The distribution ofthese samples within the angular interval determines the transferfunction of the filter. Generally, a filter with zeros is obtained for alarge number of frequencies in the transfer function, as well as amoderate damping at high frequencies. The moderate damping for highfrequencies implies that the filter permits rapid changes of the outputsignal, which is desirable when detecting misfiring. The zeros implythat the filter completely eliminates the variation of the input signalat these frequencies, but allows signals with other frequencies to pass.By choosing the selection of sample points such that the zeros lie atthe frequencies which are to be suppressed, that is, the frequencieswhich are excited by the engine type in question, the desired transferfunction is obtained.

The calculation of the indicated mean effective torque may thus,according to the invention, be carried out in several different ways fordifferent types of engines. It would also be possible to use differentmethods for the same engine, depending on the speed of the engine.

The indicated mean effective torque functions very well as a quantityfor detecting occasional misfirings. For the most difficult operatingcase for detection of misfiring, which is low load at high speed, thedifference in the indicated mean effective torque during normaloperation and in case of misfiring is up to six times greater than thegeneral noise level.

How misfiring is detected based on the indicated mean effective torqueis to a certain extent a pure matter of definition, which must bedetermined in each individual case depending on the purpose of themisfiring detection.

This is especially apparent when it comes to misfiring detection duringno-load operation and low speeds, when an internal combustion engine ofcourse exhibits very great variations in work carried out per cylinderand likewise in the indicated mean effective pressure.

Misfiring may occur both as intermittent and continuous misfiring. Thedetermination of how great a deviation from the normal indicated meaneffective pressure that is to be allowed in these cases, without beingclassified as misfiring, must in certain cases be made on the basis ofthe requirements of law, or depending on the specifications of theengine manufacturer when dimensioning the components included in theengine.

The method according to the invention, which is used for detectingoccasional misfirings, comprises comparing the indicated mean effectivetorque for a cylinder with a weighted mean of the indicated meaneffective torque of adjacent firings. If the deviation is sufficientlygreat, that firing is considered a misfiring.

A variant of the above method is included as part of the invention.According to this variant, the standard deviation of previous firings iscontinuously formed and the threshold value is expressed in a number ofstandard deviations. The difference is then a measure of the extent towhich the torque signal must deviate from a background noise which ispresent in the torque signal in this very operating case.

Within the scope of the invention, however, with the aid of mean-valueformation of the indicated mean effective torque over a suitable numberof cycles, a possibility of detecting very small differences in theindicated mean effective torque per cylinder may be obtained. Tests haveshown that a sufficient difference for being able to detect continuousmisfiring is obtained by mean-value formation over four cycles. Thismethod is especially suitable for detecting unbalance between cylindersduring normal operation. In this way it is possible to compensate forthis unbalance, for example by individual control of the quantity offuel injected into the cylinders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an explanatory sketch of a device according to theinvention for detecting misfiring of an internal combustion engine.

FIG. 2 shows an impulse response, obtained by convolution, for an FIRfilter for the second, 2.5^(th) and third tones, suitable as weightingfunction for five-cylinder engines.

FIG. 3 shows the transfer function for an FIR filter according to FIG.2.

FIG. 4 shows the sampling points for a simplified FIR filter, based on16 samples corresponding to the FIR filter from FIG. 2.

FIG. 5 shows the transfer function for a simplified FIR filter accordingto FIG. 4.

FIG. 6 shows the torque signal from a five-cylinder engine with fourmisfirings during 10 cycles, filtered by means of an FIR filteraccording to FIG. 2.

FIG. 7 shows the torque signal from the same situation as in FIG. 6,filtered by means of a simplified FIR filter according to FIG. 4.

FIG. 8 shows the transfer function for a simplified FIR filter forfive-cylinder engines, based on 32 samples.

FIG. 9 shows the equidistant 16 sample points for a simplified FIRfilter for four-cylinder engines and.

FIG. 10 shows the transfer function for a simplified FIR filteraccording to FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an explanatory sketch of a device for carrying out a methodaccording to the invention for detecting misfiring of an internalcombustion engine 1. The torque of the internal combustion engine ismeasured by means of a torque sensor 2 mounted on the crankshaft 3 ofthe internal combustion engine, and a torque signal is sampledspeed-synchronously with the aid of a sensor 4. The filtering of thetorque signal is carried out with a digital filter 5 with a finiteimpulse response which blocks or damps a selection of integer orhalf-integer multiples of the speed of the internal combustion engine,where the selection of speed multiples is substantially determined bythe number of cylinders of the internal combustion engine.

To obtain the highest possible accuracy in the indicated mean effectivetorque, the torque sensor is placed between the crankshaft and theflywheel.

If the calculation capacity does not constitute any limitation, the FIRfiltering may be carried out in conventional manner. The torque signalis sampled speed-synchronously in an angular interval around each firingpulse and the mean value thereof is formed using a weighting functionwhich for a five-cylinder engine corresponds to the impulse response ofa filter for the second, 2.5^(th) and third tones and their harmonics.Such a filter function is obtained by convoluting a second, 2.5^(th) andthird tone filter with one another. The convolution integral isapproximated by steps corresponding to the sampling step and requires asmany multiplications and additions as the number of samples whichcorresponds to the length of the impulse response.

The second-tone filter is rectangular with the value 1 for an intervalof 180° and the value 0 for the remaining part of a work cycle (twoturns, 720°). In a corresponding way, a 144° interval with the value 1corresponds to the filter of the 2.5^(th) tone, a 120° interval with thevalue 1 the third-tone filter, etc. The phase position, that is, thecentering of these intervals around an angle measured from the upperdead center at the beginning of the power stroke, may be chosen foroptimization of the result.

For a five-cylinder engine, the weighting function of the filter isshown, which has been achieved with the already-described convolutingoperation with the second, 2.5^(th) and third tones in FIG. 2, where theweighting function is shown in arbitrary units as a function of theangle relative to the upper dead center at the beginning of the powerstroke. FIG. 3 shows the transfer function, that is, the damping indecibels as a function of the frequency in speed units, for this filter.

The filter becomes especially simple for a four-cylinder in-line enginesince it only comprises the mean value of the torque signal during asuitably centered 180° interval for each cylinder.

An FIR filter with a result equal to that with the conventional method,but with a greatly reduced need of calculation capacity, may beadvantageously obtained by selecting only certain of the sampled values,for example 16 or 32 values, placed within an angular interval, whichare then summed. The distribution of these samples determines thefrequency properties of the filter, as previously shown.

An embodiment of such a filter for a five-cylinder engine based on 16samples is shown in FIG. 4, where the location of the selected samplesis shown in angular degree in relation to the upper dead center at thebeginning of the power stroke. The required sampling frequency forrealizing this filter exactly is equal to 60 samples per turn. Thetransfer function for this filter is shown in FIG. 5. The transferfunction of the filter has zeros at the second, 2.5^(th) and third tonesas well as their odd harmonics, and an additional zero at the fifthtone. FIG. 6 and FIG. 7 show the filtered torque signal for afive-cylinder engine with four misfirings during 10 cycles. In FIG. 6,the previously described conventional FIR filter, obtained byconvolution, has been used, in FIG. 7 the filter based on the 16 samplesshown in FIG. 4. The filters in these two cases have been applied asrunning filters to illustrate the identification of the misfiringcylinder. In the method according to the invention, a suitable timewithin the work cycle is chosen for each cylinder, in which misfiringgives the clearest reading after filtering. For the purpose of misfiringindication, the signals from both filters are equivalent. The fact thatthe result of the modified filter contains more high-frequency noise isbalanced by the considerably reduced extent of the calculation. With anincreased number of samples, the filter may be further improved. FIG. 8shows the transfer function of a filter for a five-cylinder engine basedon 32 samples. For a four-cylinder in-line engine, a filter which,according to FIG. 9, is based on 16 equidistant samples is very simpleand hence efficient to implement from the computational point of view.From the corresponding transfer function according to FIG. 10, it isclear that the second tone and its harmonics are eliminated.

The torque signal after the filtering, with either the conventionalmethod or the modified FIR method according to the above, gives a valuefor each cylinder during a work cycle. This value is then compared witha suitable statistical measure based on a number of work cycles, aspreviously described.

The implementation of the misfiring detector takes place in amicrocomputer system operating in real time. The filtering is carriedout digitally in the software.

For Otto engines, the misfiring detector may be combined withmeasurement of the voltage across the spark plug and its duration todetermine whether the misfiring is caused by a fault in the ignitionsystem.

What is claimed is:
 1. A method for detection of at least one ofmisfiring and unbalance between the cylinders of an internal combustionengine, said method comprising the steps of:using a torque sensormounted on a crankshaft of an internal combustion engine to measure atorque of said internal combustion engine and generate a torque signaltherefrom; sampling said torque signal speed-synchronously; performing aweighted averaging of the torque signal with a weighting function whichcancels or damps an influence of sinusoidal variations of said torquesignal with frequencies equal to a selection of integer or half-integermultiples of a speed of said internal combustion engine; wherein:saidselection of speed multiples is substantially dependent on a number ofcylinders of the internal combustion engine; and said detection is basedon a result of said averaging.
 2. A method according to claim 1, whereinsaid sinusoidal variations of the torque signal with frequencies equalto twice the speed of the internal combustion engine, and multiples ofsaid speed other than two, are cancelled or damped by the weightingfunction.
 3. A method according to claim 1, wherein at least sinusoidalvariations of the torque signal with frequencies equal to 2, 2.5 and 3times of the speed of the internal combustion engine are cancelled ordamped by the weighting function.
 4. A method according to claim 1,wherein at least sinusoidal variations of the torque signal withfrequencies equal to 3 and 4.5 times the speed of the internalcombustion engine are cancelled or damped by the weighting function. 5.A method according to claim 1, wherein at least sinusoidal variations ofthe torque signal with frequencies equal to 4, 5 and 6 times the speedof the internal combustion engine are cancelled or damped by theweighting function.
 6. A method according to claim 1, wherein theweighting averaging is carried out once during a working cycle of eachcylinder.
 7. A method according to claim 1, wherein the weightingfunction is a rectangular function of a given length, giving all samplestaken into account the same weight.
 8. A method according to claim 1,wherein the weighting function is obtained by convolution of two or morerectangular functions of different lengths.
 9. A method according toclaim 1, wherein the averaging is performed by summation of a selectionof sample points of the torque signal, said sample points beingdistributed on crankshaft angles in such a way that the averaging isadapted to cancel or damp the influence of sinusoidal variations of thetorque signal with one or a plurality of said speed multiples.
 10. Amethod according to claim 1, wherein misfiring is indicated when adeviation of an indicated mean effective torque, obtained by saidaveraging, for a cylinder from a weighted mean of the indicated meaneffective torque of adjacent firings exceeds a threshold value which maydepend on the indicated mean effective torque.
 11. A method according toclaim 10, wherein the threshold value is partly determined by acontinuously formed standard deviation of a number of preceding firings.12. A method according to claim 10, wherein a mean value of theindicated mean effective torque during a number of firings is formed foreach cylinder and used for detection of at least one of continuousmisfiring or unbalance between the cylinders.
 13. A method according toclaim 1, wherein the torque sensor is arranged between the crankshaftand a flywheel.
 14. A method according to claim 1, wherein a voltageacross a spark plug and the duration of said voltage are measured. 15.The method of claim 1, wherein said internal combustion engine is adiesel engine, and said method further comprises compensating for saidunbalance by controlling a quantity of fuel injected into the cylinders.16. A device for detection of misfiring of an internal combustion enginecomprising:a torque sensor mounted on a crankshaft of an internalcombustion engine, for measurement of a torque of the internalcombustion engine where a torque signal from said torque sensor issampled speed-synchronously; wherein:a weighted averaging of the torquesignal is performed with a weighted function which cancels or damps theinfluence from sinusoidal variations of the torque signal withfrequencies equal to a selection of integers or half-integer multiplesof the speed of the internal combustion engine; the selection of saidspeed multiples is substantially dependent on the number of cylinders ofthe internal combustion engine; and the detection is based on a resultof said averaging.
 17. A device according to claim 16, whereinsinusoidal variations of the torque signal with frequencies equal totwice the speed of the internal combustion engine, and multiples of saidspeed other than two, are cancelled or damped by the weighting function.18. A device according to claim 16, wherein at least sinusoidalvariations of the torque signal with frequencies equal to 2, 2.5 and 3times the speed of the internal combustion engine are cancelled ordamped by the weighting function.
 19. A device according to claim 16,wherein at least sinusoidal variations of the torque signal withfrequencies equal to 3 and 4.5 times the speed of the internalcombustion engine are cancelled or damped by the weighting function. 20.A device according to claim 16, wherein at least sinusoidal variationsof the torque signal with frequencies equal to 4, 5 and 6 times thespeed of the internal combustion engine are cancelled or damped by theweighting function.
 21. A device according to claim 16, wherein thedevice carries out the averaging once during a working cycle of eachcylinder.
 22. A device according to claim 16, wherein the weightingfunction is a rectangular function of a given length, giving all samplestaken into account the same weight.
 23. A device according to claim 16,wherein the weighting function is obtained by convolution of two or morerectangular functions of different lengths.
 24. A device according toclaim 16, wherein the device carries out the averaging by summation of aselection of sample points of the torque signal, said sample pointsbeing distributed on the crankshaft angles in such a way that theaveraging is adapted to cancel or damp the influence of sinusoidalvariations of the torque signal with one or a plurality of said speedmultiples.
 25. A device according to claim 16, wherein said deviceindicates misfiring when a deviation of an indicated mean effectivetorque, obtained by said averaging, for a cylinder from a weighted meanof the indicated mean effective torque of adjacent firings exceeds athreshold value which may depend on the indicated mean effective torque.26. A device according to claim 25, wherein said threshold value ispartly dependent on a continuously formed standard deviation of a numberof preceding firings.
 27. A device according to claim 25, wherein thedevice, for each cylinder, forms a mean value of the indicated meaneffective torque during a number of firings for detection of at leastone of continuous misfirings or unbalance between the cylinders.
 28. Adevice according to claim 16, wherein the torque sensor is arrangedbetween the crankshaft and a gearbox.
 29. A device according to claim16, wherein the torque sensor is arranged between the crankshaft and aflywheel.
 30. A device according to claim 16, wherein the torque sensoris a magnetoelastic transducer.
 31. A device according to claim 16,wherein the device also comprises means for measurement of a voltageacross a spark plug and the duration of said voltage.